1. Field of the Invention
The present invention relates to a communication system comprising a plurality of communication terminals and a data communication unit.
2. Related Background Art
A communication system comprising a facsimile unit and a telephone set as shown in FIG. 4 has been known. In such a system, the telephone set used as a terminal device either has a calling function or does not have the calling function.
In the system in which the telephone set has the calling function, a telephone line 2a is connected to a CML (communication line) relay of a NCU (network control unit) of a facsimile unit 2 and the line is switched between a facsimile transmitter/receiver and a telephone set 4 by the CML relay. In a manual transmission mode of such a system, the CML relay is switched to the telephone set 4 and a selection signal is sent by a calling device of the telephone set 4. In the calling mode, the telephone set 4 is directly connected to the telephone line 2a and the facsimile unit 2 is not connected. As a result, the dialing signal sent to the line cannot be recognized by the facsimile unit 2.
In a system in which the telephone set 4 does not have the calling function and the calling function provided in the facsimile unit 2 is used or a system in which the facsimile unit 2 has dialing means such as a ten-key and a calling circuit, the facsimile unit 2 can recognize the telephone number when the telephone number is entered by the facsimile unit 2, but it cannot recognize the telephone number when it is manually sent by the telephone set 4.
In the prior art system, when the manual operation is performed done in the telephone set, the facsimile unit cannot recognize the telephone number. Accordingly, it is not possible to check whether the operator has called the desired station by comparing the entered telephone number with the telephone number of the destination station transferred in the facsimile protocol.
In the prior art system, there is rarely a possibility of misconnection by a malfunction of a communication network. On the other hand, as communication is liberalized, there is a possibility that different communication equipment are connected to different communication apparatus and so the possibility that communication is made among equipment of different manufacturers increases. Also, the status of the communication network is not always constant. In these situation, a possibility of misconnection is not zero.
In speech communication, the speech communication is not attained in the case of misconnection and the misconnection may not raise a serious problem although the communication cost increases and the operation by a user is complex. In the case of facsimile transmission, however, information of a text may be transmitted to a misconnected wrong station if the station has a facsimile unit. Thus, a text of high confidentiality cannot be transmitted with reliability.
In order to avoid the above problem a system is required in which the calling function is under the control of a control unit which is common to communication apparatus including a telephone set as a terminal and a facsimile unit so that the entered telephone number and the telephone number transferred from the destination station are compared.
Recently, there are increasing cases where not only the telephone set but also the facsimile unit or data communication apparatus are connected to the telephone line. When a communication apparatus other than a voice terminal such as a facsimile unit or a data communication apparatus is used, it is usual to construct a system which allows both voice communication and predetermined communication by the communication apparatus.
In order to allow both voice communication and the predetermined communication operation, one of several methods may be used. In one method, different lines are provided for the voice communication and the predetermined communication and they are called by different telephone numbers. In this system, since the communication apparatus is in many cases connected together with the telephone set, it is possible to send and receive calls for speech by manual operation, but when the communication apparatus is in the auto-receive mode, it is not possible to receive the call for speech.
In one system, a representative number is used to connect the communication apparatus. As is well known, in the system which uses the representative number, if the line corresponding to the representative number is busy, a line corresponding to another telephone number is used for communication. The following two methods may be used to connect the communication apparatus to the system which uses the representative number.
In one method, a line control unit detects a voice signal from a calling station each time it receives a call in order to determine whether the calling station intends to perform voice or non-voice communication, and selects an appropriate terminal device. In this method, any number may be used to attain both the voice communication and the communication of the predetermined type but the system cost is very high because it requires a voice recognition apparatus. Further, if all communication apparatus which are connected are busy, the communication of the predetermined type is not attained even if the calling station desires the communication.
In a communication system which accommodates a plurality of telephone lines by a representative number, a communication apparatus is allotted to a specific telephone number and the non-voice communication instead of the voice communication is made if the number corresponding to the communication apparatus is called. In this system, when a call is sent by the representative telephone number and the lines corresponding to other telephone numbers than the representative number are busy, the line to which the communication apparatus is connected is selected. In such a case, speech is not attained even if the calling station desires it.
In the facsimile unit, speech may be assured by phone reservation even if the communication apparatus is selected but such a control requires a complex operation.
In a push button telephone system which has a hold function, if an external line is held for more than a predetermined time, a hold alarm sound is generated to prevent invalid hold. The hold alarm sound is generated at a predetermined time interval from the start of external line hold. The predetermined time may be set arbitrarily, for example, 1, 2, 3, 4 or 5 minutes. The duration of the hold alarm sound is generally set to approximately ten seconds.
In such a system, when the external line is held, it is not possible to determine how long the time has elapsed since the external line was held, by merely hearing the hold alarm sound. If the operator does not respond while the external line is held, the external line is kept held for no reason.
The communication systems comprising a plurality of voice terminals and a facsimile unit are disclosed in U.S. patent appln. Ser. No. 823,286 filed on Jan. 28, 1986, which issued as U.S. Pat. No. 4,825,461 on Apr. 25, 1989; U.S. patent appln. Ser. No. 813,514 filed on Dec. 26, 1985, which issued as U.S. Pat. No. 4,785,355 on Nov. 15, 1988; U.S. patent application Ser. No. 453,364 filed on Dec. 20, 1989, which was a continuation of U.S. patent appln. Ser. No. 108,915 filed on Oct. 15, 1987, now abandoned; and U.S. patent appln. Ser. No. 106,783 filed on Oct. 13, 1987 pending, and U.S. patent appln. Ser. No. 134,806 filed on Dec. 18, 1987, which issued as U.S. Pat. No. 4,870,680 on Sept. 26, 1989.